Our interest is to define the mechanism by which thymus-derived (T) lymphocytes affect the response of thymus-independent (B) lymphocytes to antigen. We have recently demonstrated that T lymphocytes have the ability to suppress the maturation of B lymphocyte precursor cells to antibody secreting cells. In particular, the suppressor cells act most effectively on B cells with high affinity receptors. The suppressor function is clearly due to theta-bearing cells with carrier specificity which act solely on IgG antibody producers, and not on IgM antibody producers. We are currently defining the stage of B cell differentiation which is susceptible to suppression. In addition, we will examine the suppressive function more fully with respect to its relation to helper cells. Bibliographic references: Davie, J.M. and Paul, W.E.: "Role of T lymphocytes in the humoral immune response. I. Proliferation of B lymphocytes in thymus-deprived mice." J. Immunol. 113: 1438-1445, 1974; Claflin, J.L. and Davie, J.M.: "Clonal nature of the immune response to phosphorylcholine. III. Species-specific binding characteristics of rodent anti-phosphorylcholine antibodies." J. Immunol. 113: 1678-1684, 1974.